1. Field of the Invention
This invention relates to a diode limiter, especially, to a diode limiter used for a pulse radar apparatus or the like.
2. Description of the Related Art
In a pulse radar apparatus, high power transmission pulses generated in a transmitter are emitted from an antenna. Echoes reflected from an object are received by the same antenna and led to a receiver. These transmitting and receiving operations are continuously repeated.
Therefore, in the transmitting operation, the antenna should be connected to the transmitter, while, in the receiving operation (i.e., in the time interval other than for the transmitting operation), it should be connected to the receiver. Thus, a switching means is required in the pulse radar apparatus for switching the connection of the antenna from the transmitter to the receiver and vice versa.
The transmission-receiving switching means as mentioned above functions to switch the connection of the antenna from the transmitter to the receiver and vice versa and further functions to protect the head of the receiver from high power pulse signals such as the transmitting pulse signals emitted from the same station, excessively powerful echoes reflected from an object located nearby, or a transmitting pulse emitted from another adjacent radar station.
Recently, reliable protection has been required especially for a pulse radar apparatus having a high performance receiver head.
In the prior art, to obtain such a switching function and a protecting function in the pulse radar apparatus, a discharge tube such as a TR tube (transmit-receive tube) and an ATR tube (anti-TR tube) were used.
However, these kinds of discharge tubes had the disadvantage that there was usually a certain time lag between when the tube was turned ON and when the discharge operation thereof was started, whereby a part of the rising portion of the transmission pulse was leaked to the receiver, causing insufficient protection.
Further, these kinds of discharge tubes have another disadvantage in that the short range detection distance is considerably increased, since there is necessarily recovery time when the discharge operation is stopped.
Therefore, a diode limiter which can operate at rather higher speed has recently been used instead of a discharge tube.
Conventional diode limiters as mentioned above are shown in FIGS. 5(a) to (b), for example.
In FIG. 5(a), a carrier 1 and an MIC (microwave integrated circuit) substrate 2 are provided. A line 3 having a characteristic impedance of 50 .OMEGA. is formed on a surface of the MIC substrate 2.
One end thereof, i.e., an input terminal A, is connected to an antenna and a transmitter, while the other end thereof, i.e., an output terminal B, is connected to a receiver.
On the MIC substrate 2, matching circuits 4 and 5 are provided with the 50 .OMEGA. line 3. First ends of the circuits 4 and 5 are connected to the line 3. PIN diodes 6 and 7 are connected to the second opposite ends of the matching circuits 4 and 5.
FIG. 5(b) shows an equivalent circuit corresponding to the diode limiter shown in FIG. 5(a).
In the construction as shown in FIGS. 5(a) and 5(b), when a high power electric pulse signal is applied to the input terminal A, the PIN diode 6, connected through the matching circuit 4 to the input terminal A, is made conductive and becomes a low impedance. Therefore, the matching line 4 is short-circuited at the second end thereof.
In this situation, when the length of the matching line 4 defined from the connecting point (C), between the matching circuit 4 and the 50 .OMEGA. line 3, to the point which is short circuited on the end of the line, is set to .lambda./2 or a length corresponding to a whole multiple of .lambda./2, the connecting point (C) is short-circuited by the impedance of the PIN diode 6, whereby passage of the high power electric pulse signal to the output B is prevented.
However, since the impedance of the PIN diode never becomes zero, a small amount of power leaks therethrough and is actually transmitted through the point (c) to the output terminal B.
But this leaked power can be prevented from being transmitted to the output B by a low impedance condition of the connecting point (D), formed between the matching circuit 5 and the 50 .OMEGA. line 3, which is the next stage of the contacting point (C). Therefore, the level of the power at the output terminal B is restricted to a level which is sufficient for the receiver to be protected.
Conversely, when a weak receiving signal is applied to the input terminal A, the PIN diodes 6 and 7 are not made conductive and the matching lines 4 and 5 are open at their ends. As a result, the matching circuits 4 and 5 and the PIN diodes 6 and 7 appears as high impedance with respect to the 50 .OMEGA. line 3.
Accordingly, the receiving signal can be transmitted to the output terminal B with low loss.
That is, when protecting the receiver from a high power transmitted pulse, signal, the input terminal A and the output terminal B of the 50 .OMEGA. line 3 are isolated from each other by the conduction of the PIN diodes 6 and 7, while when a receiving operation is carried out, the input terminal A and the output terminal B of the 50 .OMEGA. line 3 are connected to each other with low loss without the PIN diode being made conductive.
FIGS. 6(a) and (b) show another conventional diode limiter embodiment wherein the PIN diodes 6 and 7 are directly connected to the 50 .OMEGA. line 3.
The main object of such a diode limiter is to reduce the expected maximum power applied to the input terminal A to a level sufficient to protect the receiver head. Usually, the transmitting power emitted from the same station is given as the expected maximum power.
On the other hand, the withstand power of the receiver head is different depending on the radar system to which the receiver head belongs.
Accordingly, the diode limiter is required to have an isolation characteristics tailored to individual system specifications.
In the conventional limiter shown in FIGS. 5(a) to (b) , it is required to adjust the characteristics of the PIN diodes 6 and 7 in accordance with the system used to satisfy the required isolation characteristic in the system, but a cost problem arises.
To overcome this problem, a separate method has been used in which the circuit as shown in FIG. 7 is added to the circuit shown in FIGS. 5 and 6.
In FIG. 7, a resistor 8 and an RF (radio frequency) coil 9 affording a direct current (DC) return circuit for the 50 .OMEGA. (signal) line 3 are provided. A bias voltage E.sub.v is applied across the resistor 8.
In this construction, a forward voltage of E.sub.v is always applied to the 50 .OMEGA. signal line 3, therefore the ON-OFF. characteristics of the PIN diode connected to the 50 .OMEGA. signal line 3 can be controlled by adjusting the value of the forward voltage E.sub.v.
Accordingly, an isolation characteristic suitable for individual system specifications can be obtained.
However, in the embodiment shown in FIG. 7, since a resistor 8 is provided in the direct current (DC) return circuit for the 50 .OMEGA. signal line 3, the resistance value of the resistor 8 must be set at an extremely low value such as 3 .OMEGA. to pass the DC current smoothly in the return circuit. Therefore, another problem arises in that a bias current E.sub.i passing through the resistor 8, when the power source is E.sub.v, becomes large, for example, about 150 mA, which is disadvantageous in terms of the power consumption.
To overcome this problem, the following countermeasures are provided:
(1) As shown in FIG. 8, a pickup probe 12 and a PIN diode P are provided inside a waveguide G. The ON-OFF characteristic of the PIN diode P is controlled by a compulsory bias voltage generated by a DC amplifier 14 in accordance with an output of the pickup probe 12, input through a detector diode 13 to the amplifier 14.
(2) As shown in FIG. 9, a PIN diode P is provided in the waveguide G. A directional coupler 15, a RF amplifier 16, and a detector diode 17 are provided in the vicinity of the waveguide G. The ON-OFF characteristic of the PIN diode P is controlled so that the power taken out by the directional coupler 15 is amplified by the RF amplifier 16 and the output of the amplifier 16 is applied to the PIN diode P under the control of the detector diode 17.
However, those countermeasures require a DC amplifier, and a RF amplifier respectively; thus, the problem arises that the construction of each diode limiter becomes complicated.